1. Field of the Invention
The present invention relates to a motor with a speed reducer for use with electrical equipment, and more particularly to such an apparatus having a reversal prevention mechanism use with a power window system of a car.
2. Description of the Related Art
It is known to provide a motor with a speed reducer for use with a power window system of a car. The speed reducer includes a worm gear mechanism that has a reversal prevention function so as to prevent reversal due to an external force. However, when the speed reducer is used to drive the power window system, the window can be forced open by applying a strong downward force to the window. Thus, in order to deter crime, a separate reversal prevention mechanism is provided to compliment the reversal prevention function of the worn gear mechanism. However, conventional reversal prevention mechanisms are undesirable since they include many parts and are difficult to assemble.
Therefore, Japanese Unexamined Patent Publication No. 61-36476, discloses an apparatus to brake the rotation of a motor shaft, that is integrated with a worm gear, by a frictional force generated by an end spacer. The end spacer is provided for an end face of the motor shaft. A thrust force applied to the motor shaft is communicated to the end spacer. The end spacer thereby presses and contacts the inner wall of a case (motor yoke or speed reduction case).
The yoke for supporting an end of the motor shaft is made of a very ferromagnetic metal because it must form a magnetic path. However, the speed reduction case that houses the worm gear mechanism is typically made of a lightweight resin. Therefore, a metallic plate is provided at the thrust receiving portion of the speed reduction case in order to prevent the speed reduction case from abrading due to the friction with the end spacer and from when the end spacer is forcibly brought into contact with the metallic plate.
A problem arises in that frictional heat is produced between the end spacer and the metallic plate. The frictional heat is transmitted to the metallic plate which has a heat conductivity larger than that of the plastic end spacer. Therefore, the portion of the speed reduction case to which the metallic plate is set can be softened by the frictional heat, depending on the operating condition.
Additionally, a setting hole for preventing rotation must be formed by inserting the end spacer at the front end of the motor shaft. Therefore, it is difficult to form the setting hole when the front-end diameter of the motor shaft is small, which makes assembly impossible. Thus, the industry lacks a speed reducer having a reversal prevention mechanism which is simple to manufacture and to assemble. The industry also lacks a reversal prevention mechanism which minimizes damage to the speed reduction case due to frictional heat.